EFFECTS OF BODY-SIZE, BODY DENSITY, GENDER AND GROWTH ON UNDERWATER TORQUE

Two forces act on a human body motionless in water: weight (W) and buo yancy (B). They are applied to the center of mass (CM) and to the cent er of volume (CV) of the subject, respectively. CM and CV do not coinc ide; this generates a torque that is a measure of the tendency of the upper part of the body to rise, rotating around its center of mass. To quantify this tendency, Pendergast & Craig defined 'underwater torque ' (T') as the product of the net force with which the feet of a subjec t lying horizontally in water tend to sink, times the distance between the feet and the center of volume of the lungs. In this paper we have investigated: (a) the relationships between T' and body weight (BW), height (H), body surface area (BS), body density (ED) and leg density (LD) in a group of 30 subjects (group A, 14 females and 16 males, age range 16-50 years); and (b) the effect of gender and growth on T' in a group of 110 subjects (group B, 67 girls and 43 boys, age range 12-17 years). In group A, T' was found to be linearly related with BW (r=0. 833, P<0.001), H (r=0.803, P<0.001), BS (r=0.866, P<0.001), ED (r=0.61 7, P<0.001) and LD (r=0.549, P<0.005). A multiple linear regression an alysis showed that BS and ED explained about 85% of the variability of T' (r(2)=0.85). In group B, T' was found to increase linearly with ag e (r=0.47, P<0.01), the increasing rate being three times higher in bo ys compared with girls. As a consequence, the T' ratio between boys an d girls increased with age, from 1.69 at 13 years to 2.04 at 16 years.